Certain children in certain situations behave as if they have a bias to keep word references mutually exclusive, i.e., a bias to interpret a new word so that it has no referents in common with familiar words. The word learning of children who have a mutual exclusivity bias should be subject to three general constraints-a constraint on the interpretation of an ambiguous new word; a constraint on the reaction to the use of a new word for an old referent; and a constraint on the generalization of words. The proposed research will address seven basic questions about the nature of each of these constraints. These questions are: (1) How is children's word learning aided by these constrains? (2) How are errors that should result from these constrains avoided? (3) What are the cognitive prerequisites of the constraints? (4) How do the constraints change with development? (5) What causes the developmental change? (6) What are the temporal dynamics of the constraints? (7) How do the constraints vary with respect to word type? Ten studies will be conducted. Each study will address at least one of the basic questions about at least one of the constraints. Study 1 will provide information about questions (3) and (4) by comparing adherence to the constraint on word generalization with appearance-reality confusions in 3- and 5-year-olds. Studies 2 and 3 will provide information about all questions except (6) by examining the effects of token novelty, referent typicality, and word distinctiveness on the adherence of 2-, 3-, and 4-year-olds to the disambiguation constraint for nouns and verbs. Study 4 will provide information about questions (6) and (7) by examining the effect of repeated exposure to new nouns and verbs on the adherence of 2- and 4-year-olds to the constraint on reactions to the use of a new word for an old referent. Studies 5 and 6 will provide information about all questions except (6) by examining the effects of speaker constancy and credibility on the adherence of 2-, 6-, 22-, and 19-year-olds to two of the constraints on nouns and adjectives. Studies 7 and 8 will provide information about all questions by examining the effects of correction requirement, timing of new name introduction, and memory cues on the adherence of 2- and 4-year-olds to two of the constraints on nouns and adjectives. Studies 9 and 10 will provide information about all questions except (6) by examining the ease with which 3-, 6-, 11- , and 19-year-olds learn and remember pairs of new words that have either mutually exclusive, nested, or overlapping reference.